Dan, if you look at my initial response, you'll see that one 4 ohm speaker will present a totally different load to an amplifier than another 4 ohm speaker. The impedance varies with frequency. A speaker that's 4 ohms at 20 Hz may make an amp puke, while a speaker that's 4 ohms at 1 kHz may not be a problem. Then you have the properties of inductance, capacitance, weird phase angles, inefficiency, etc. You are looking at but one parameter. It's almost like asking "What car gets better mileage...a brown one or a white one?...or a Toyota??" ;)
h/k comment: Historically, harman/kardon receivers have been able to handle tough loads, but the current universal trend in receivers is to put out as many "watts-per-dollar" as possible into 8 ohms. Most new receivers won't tolerate speakers that dip below 4 ohms or that have a nominal 6 ohm impedance. I suspect the new h/k stuff is still better-than-average in this regard.